A whale of an asteroid has gone missing. The 270-metre space rock known as 2000 EM26 was slated to skim past Earth early on 18 February, coming within 3.4 million kilometres of our planet. But when a robotic telescope service trained its eye on the predicted position, the asteroid was nowhere to be found.

Astronomers coordinating the telescope service, called Slooh, have nicknamed the elusive asteroid Moby Dick after the fictional white whale, and have issued a call to amateur sky-watchers to help hunt it down.

It is not uncommon for asteroids to go missing and it is unlikely that Moby Dick now poses a danger to Earthlings. But its apparent disappearance highlights just how poor Earth is at asteroid surveillance.

So what has happened to asteroid 2000 EM26? The most likely scenario is that it is on a very different path from its expected trajectory, and the telescope wasn't looking in the right place, says Slooh CEO Michael Paolucci. Generally, astronomers try to predict an asteroid's trajectory by looking at how light reflects off its surface. This tells them what it is probably made of, how it is spinning and where it might go next.

Asteroid 2000 EM26 was discovered 14 years ago and has not been seen since, so astronomers have limited information about it. Not knowing enough about how the asteroid rotates makes it hard to know how other forces, likeradiation pressure from sunlight, might nudge the rock onto a different trajectory.

Generally, it is not unusual to be uncertain about any asteroid's future whereabouts, says Paolucci. Even before Moby Dick failed to show, the telescope's operators were deliberating with a partner in Dubai about exactly where to look. "It's a major chore figuring out how to reacquire asteroids," says Paolucci. "It's almost like discovering them all over again."

Sometimes asteroids are simply too dark in colour to see easily, making them difficult to find again with visible-light telescopes like Slooh. This might explain how a big asteroid like Moby Dick can remain elusive even as it makes its closest approach to Earth."One possibility here is that the asteroid is right where we think it is. It might just be really faint," says Amy Mainzer of the Jet Propulsion Laboratory in Pasadena, California. Although telescopes that look at other wavelengths of light might be able to see it, they haven't looked yet.